1. Field of the Invention
The present invention relates to an automatic blood-pressure measuring apparatus which automatically measures a blood pressure of a living subject based on a physical signal obtained when a pressure in a cuff is decreased.
2. Related Art Statement
There is known an automatic blood-pressure measuring apparatus of a type which includes a cuff inflatable to press a portion of a living subject and measures a blood pressure of the subject based on a physical signal, such as a pulse wave or Korotkoff sounds, that is continuously obtained when the pressure of the cuff is slowly decreased.
The pressure of the cuff needs to be increased, in advance, before the cuff pressure is slowly decreased. It is the most appropriate for the increasing of the cuff pressure to be stopped at the lowest possible pressure at which, however, the cuff can completely stop the flow of blood through the portion of the subject around which the cuff is wound. The lowest possible pressure needs to be higher than a systolic blood pressure of the subject.
However, individual living subjects may have different systolic blood-pressure values. Hence, the conventional automatic blood-pressure measuring apparatus quickly increases the pressure of the cuff up to a prescribed increase-end pressure (e.g., 180 mmHg) that is sufficiently higher than an average systolic blood pressure. However, for some subjects, the prescribed increase-end pressure may be too low to measure their systolic blood-pressure values. In addition, for other subjects, the prescribed increase-end pressure may be unnecessarily high to measure their systolic blood-pressure values, causing them to feel discomfort because the high pressure of the cuff is applied in an unnecessarily long time.
It is therefore an object of the present invention to provide an automatic blood-pressure measuring apparatus which automatically measures a blood pressure of a living subject based on a physical signal obtained when a pressure in a cuff is decreased after having been increased to an increase-end pressure and which can increase the cuff pressure to the increase-end pressure appropriate for a systolic blood pressure of the subject.
The above object has been achieved by the present invention. According to the present invention, there is provided an apparatus for automatically measuring a blood pressure of a living subject, comprising an inflatable cuff which is adapted to be wound around a portion of the subject; an increase-end-pressure determining means for determining, based on a cuff pulse wave occurring to the cuff when a pressure in the cuff is increased at a first rate, an increase-end pressure to which the pressure of the cuff is increased at the first rate, at which the increasing of the pressure of the cuff is ended, and from which the pressure of the cuff is decreased at a second rate lower than the first rate; and a blood-pressure determining means for determining a blood pressure of the subject based on a physical signal obtained from the subject when the pressure of the cuff is decreased at the second rate from the increase-end pressure.
According to this invention, the increase-end-pressure determining means determines, based on a cuff pulse wave occurring to the cuff when the pressure of the cuff is increased at a first rate, an increase-end pressure at which the increasing of the pressure of the cuff is ended. Since a waveform of the cuff pulse wave occurring to the cuff when the cuff pressure is increased changes according to a relationship between cuff pressure and subject""s blood pressure, the thus determined increase-end pressure is appropriate for the subject whose systolic blood pressure may differ from those of other subjects.
Preferably, the increase-end-pressure determining means determines the increase-end pressure based on a rate of decrease of an after-peak-point portion of at least one heartbeat-synchronous pulse of the cuff pulse wave occurring to the cuff when the pressure of the cuff is increased at the first rate, the after-peak-point portion starting with a peak point of the heartbeat-synchronous pulse.
The respective decrease rates of respective after-peak-point portions of respective heartbeat-synchronous pulses of the cuff pulse wave increase as the pressure of the cuff is increased in a range lower than a systolic blood pressure of the subject, and largely decrease as the cuff pressure is increased over the systolic blood pressure. Therefore, the increase-end-pressure determining means may determine the increase-end pressure based on a pressure of the cuff when one of the respective decrease rates of respective after-peak-point portions of respective pulses of the cuff pulse wave significantly largely decreases, or may determine, as the increase-end pressure, a pressure greater by a prescribed additional pressure than a pressure of the cuff when one of the respective decrease rates of the respective after-peak-point portions is greater than a prescribed decrease-rate value.
Preferably, the increase-end-pressure determining means determines the increase-end pressure based on whether a peak portion of at least one heartbeat-synchronous pulse of the cuff pulse wave occurring to the cuff when the pressure of the cuff is increased at the first rate has a small-projection shape.
In the state in which the cuff completely stops the flow of blood through the portion of the subject around which the cuff is wound, the peak portion of each heartbeat-synchronous pulse of the cuff pulse wave occurring to the cuff has a small-projection shape. Therefore, the increase-end-pressure determining means can determine the increase-end pressure based on whether the peak portion of one heartbeat-synchronous pulse of the cuff pulse wave has the small-projection shape.